What is the male form of mare?

Well for a male horse that has not been castrated (fixed) they would be called stallions, if they were castrated they would be called geldings.
A mare is a female horse aged five or older (some say three or older). The male counterpart is stallion or gelding. Most male horses are gelded as yearlings to prevent them from breeding and from developing aggressive traits.

A younger mare is called a filly. The male counterpart is a colt.
A mare is a female horse, and a stallion is a male horse.
a mare is a female horse and a male one is a stallion